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BigBattery

BigBattery ETHOS + EG4 FlexBOSS21 Kit | 10.24–30.72 kWh | Split-Phase Home ESS

SKU K0241

Regular price: $6,460.00

Regular price: $6,460.00

✅ Order by 12:00 PM EST and processing starts the same day (ships within 1–2 business days). Free, insured US shipping — delivery in 4–7 business days. Oversized items ship via LTL freight and we coordinate delivery with you by phone.

Product variants

Quantity

OffGrid Horizon Family Owned and Operated

🏔️ OffGrid Horizon is family-owned & off-grid obsessed. Read our story →

Shipping: how fast & how it works

Free standard shipping on every US order — no shipping charges at checkout, no hidden fees, and every shipment is insured against loss or damage.

Processing: 1–2 business days. Order before 12:00 PM EST and processing begins the same day.
Delivery: 4–7 business days via USPS, UPS or FedEx. Tracking is emailed the moment your order ships.
Large items (solar kits, batteries, shelters): delivered by LTL freight carriers such as FedEx Freight, Old Dominion or XPO. The carrier calls ahead to schedule a delivery window — please inspect the pallet and note any damage on the delivery receipt before signing, and we’ll take care of the rest.

Questions about delivery to your location? Call (302) 496-3554 (Mon–Fri, 9am–4pm EST) — we’re happy to check before you order.

What is your Return Policy?

At OffGrid Horizon, we want every customer to shop with confidence. If something doesn’t go as planned, we’ve made returns and refunds simple and fair. Here’s how it works:

1. Return Window

You can request a return within 30 days of receiving your order. Unfortunately, we’re unable to accept returns after that time frame.

2. Who Pays for Return Shipping?

If your item arrives damaged, defective, or incorrect, we’ll cover the return shipping cost — and you’ll receive a full 100% refund or replacement, including any original shipping costs.
If you’re returning an item because you changed your mind, you’ll be responsible for return shipping, and a 25% restocking fee applies.

3. Damaged or Defective Items

Received something broken or not working properly? Contact us within 30 days. We’ll provide a prepaid return label and once the item is returned and inspected, we’ll issue either a full refund or a replacement.

4. Changed Your Mind?

If you no longer want the item, you can return it within 30 days — as long as it’s unused, unopened, in its original packaging, and in resellable condition.

5. Refund Processing

Once we receive and approve your return, we’ll send a confirmation email and issue a refund to your original payment method within 7 business days. Please allow additional time for banks or credit card companies to process and reflect the refund.

How to start a return: email sales@offgridhorizon.com or call (302) 496-3554 (Mon–Fri, 9am–4pm EST) with your order number — we’ll send return instructions and your RMA right away.

Product Description

The step-up configuration: stackable BigBattery ETHOS 48V LiFePO4 battery modules paired with the EG4 FlexBOSS21 hybrid inverter — 21 kW of solar input and 16 kW of continuous split-phase AC output across four MPPT channels, with remote Wi-Fi monitoring built in. Configure 10.24 to 30.72 kWh of storage to match your backup goals, from essential circuits to whole-home autonomy.

What's in this kit

1 × EG4 FlexBOSS21 hybrid solar inverter — 21 kW PV input, 16 kW continuous split-phase 120/240V output, 4 MPPT channels, grid-tied / off-grid / backup operating modes, and app-based Wi-Fi monitoring
Your choice of 2, 3, 4, or 6 × BigBattery ETHOS 48V 5.12 kWh LiFePO4 modules — 10.24, 15.36, 20.48, or 30.72 kWh of stackable, tower-style storage
Closed-loop communication between battery and inverter for accurate state-of-charge reporting and coordinated charging

Choosing your capacity

2 modules (10.24 kWh) backs up essential circuits through an evening outage. 3–4 modules (15.36–20.48 kWh) carries most households overnight, including a well pump and moderate 240V loads. 6 modules (30.72 kWh) is the extended-autonomy configuration — the right pairing if you chose the FlexBOSS21 for its output headroom, since heavier loads drain smaller banks quickly. You can also start smaller and stack additional ETHOS modules later.

Key features

16 kW of continuous output for larger homes and heavier simultaneous loads — central air, well pump, range, and shop tools at the same time
Four MPPT inputs handle complex arrays spread across multiple orientations
Safety-certified battery platform: ETHOS modules are built on UL1973-listed cells with UL9540A-tested design
Remote monitoring: app-based visibility of production and battery state via Wi-Fi
Scalable both ways: stack additional ETHOS modules, or parallel additional FlexBOSS inverters

Specifications

Battery platform	BigBattery ETHOS, 48V LiFePO4, 5.12 kWh per module
Capacity options	10.24 / 15.36 / 20.48 / 30.72 kWh (2, 3, 4, or 6 modules)
Battery certifications	UL1973-listed cells; UL9540A-tested design
Inverter	EG4 FlexBOSS21 hybrid
Solar (PV) input	Up to 21 kW across 4 MPPT channels
AC output	16 kW continuous, split-phase 120/240V
Operating modes	Grid-tied, off-grid, battery backup
Monitoring	Remote Wi-Fi app monitoring
Expandable	Yes — add ETHOS modules or parallel inverters

Warranty & support

Battery: ETHOS modules are covered by BigBattery's 12-year manufacturer warranty, backed by lifetime USA-based technical support. Inverter: the EG4 FlexBOSS21 carries a 10-year full parts-or-product replacement manufacturer warranty; EG4 requires warranty registration within the first year of purchase (or proof of purchase from an authorized distributor) to keep coverage valid.

Common questions

FlexBOSS21 or FlexBOSS18?

Choose the FlexBOSS21 for 16 kW of output and a fourth MPPT — worth it for larger homes, heavy 240V loads, or complex arrays. If your loads are moderate, the FlexBOSS18 kit delivers the same battery platform at a lower price.

Do I need an electrician?

Yes — a hybrid inverter interconnects with your main panel and, in grid-tied modes, with the utility. Installation should be performed by a licensed electrician, and home energy storage installations are subject to local permitting.

Are solar panels included?

No. Panels, racking, and array wiring are sized to your site and sold separately. The system also operates as battery backup without solar. Select your module count from the Battery Configuration menu — pricing shown is for the complete kit.